CN114982008A - Fluorine-containing positive electrode active material for lithium secondary battery and lithium secondary battery comprising same - Google Patents
Fluorine-containing positive electrode active material for lithium secondary battery and lithium secondary battery comprising same Download PDFInfo
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- CN114982008A CN114982008A CN202080089922.8A CN202080089922A CN114982008A CN 114982008 A CN114982008 A CN 114982008A CN 202080089922 A CN202080089922 A CN 202080089922A CN 114982008 A CN114982008 A CN 114982008A
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- positive electrode
- electrode active
- active material
- mixed layer
- lithium
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- 239000007774 positive electrode material Substances 0.000 title claims abstract description 140
- 229910052744 lithium Inorganic materials 0.000 title claims abstract description 117
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 title claims abstract description 115
- YCKRFDGAMUMZLT-UHFFFAOYSA-N Fluorine atom Chemical compound [F] YCKRFDGAMUMZLT-UHFFFAOYSA-N 0.000 title claims abstract description 52
- 239000011737 fluorine Substances 0.000 title claims abstract description 52
- 229910052731 fluorine Inorganic materials 0.000 title claims abstract description 52
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims abstract description 229
- 150000003624 transition metals Chemical class 0.000 claims abstract description 79
- 229910052723 transition metal Inorganic materials 0.000 claims abstract description 78
- 229910052759 nickel Inorganic materials 0.000 claims abstract description 51
- 230000003647 oxidation Effects 0.000 claims abstract description 20
- 238000007254 oxidation reaction Methods 0.000 claims abstract description 20
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims abstract description 5
- 229910052760 oxygen Inorganic materials 0.000 claims abstract description 5
- 239000001301 oxygen Substances 0.000 claims abstract description 5
- -1 nickel Chemical class 0.000 claims abstract description 4
- 239000011163 secondary particle Substances 0.000 claims description 56
- 239000011164 primary particle Substances 0.000 claims description 48
- 239000013078 crystal Substances 0.000 claims description 29
- 239000011572 manganese Substances 0.000 claims description 29
- HBBGRARXTFLTSG-UHFFFAOYSA-N Lithium ion Chemical compound [Li+] HBBGRARXTFLTSG-UHFFFAOYSA-N 0.000 claims description 18
- 229910001416 lithium ion Inorganic materials 0.000 claims description 18
- 229910052748 manganese Inorganic materials 0.000 claims description 18
- 239000011248 coating agent Substances 0.000 claims description 15
- 238000000576 coating method Methods 0.000 claims description 15
- 238000004833 X-ray photoelectron spectroscopy Methods 0.000 claims description 13
- 229910052782 aluminium Inorganic materials 0.000 claims description 13
- 239000002245 particle Substances 0.000 claims description 13
- 238000002441 X-ray diffraction Methods 0.000 claims description 10
- 229910001428 transition metal ion Inorganic materials 0.000 claims description 9
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims description 8
- 229910017052 cobalt Inorganic materials 0.000 claims description 8
- 239000010941 cobalt Substances 0.000 claims description 8
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 claims description 8
- 238000002524 electron diffraction data Methods 0.000 claims description 7
- 229910003002 lithium salt Inorganic materials 0.000 claims description 5
- 159000000002 lithium salts Chemical class 0.000 claims description 5
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 4
- 229910002804 graphite Inorganic materials 0.000 claims description 4
- 239000010439 graphite Substances 0.000 claims description 4
- 238000001228 spectrum Methods 0.000 claims description 4
- 239000008151 electrolyte solution Substances 0.000 claims description 3
- 230000005855 radiation Effects 0.000 claims description 3
- 230000009467 reduction Effects 0.000 claims description 3
- 239000007784 solid electrolyte Substances 0.000 claims description 3
- 238000003487 electrochemical reaction Methods 0.000 claims description 2
- 239000007772 electrode material Substances 0.000 abstract 1
- 239000010410 layer Substances 0.000 description 135
- 230000000052 comparative effect Effects 0.000 description 53
- 239000003792 electrolyte Substances 0.000 description 20
- 238000010438 heat treatment Methods 0.000 description 18
- DDFHBQSCUXNBSA-UHFFFAOYSA-N 5-(5-carboxythiophen-2-yl)thiophene-2-carboxylic acid Chemical compound S1C(C(=O)O)=CC=C1C1=CC=C(C(O)=O)S1 DDFHBQSCUXNBSA-UHFFFAOYSA-N 0.000 description 13
- XLYOFNOQVPJJNP-UHFFFAOYSA-M hydroxide Chemical compound [OH-] XLYOFNOQVPJJNP-UHFFFAOYSA-M 0.000 description 13
- 230000001351 cycling effect Effects 0.000 description 11
- 150000002222 fluorine compounds Chemical class 0.000 description 11
- 239000002905 metal composite material Substances 0.000 description 11
- 238000007086 side reaction Methods 0.000 description 11
- 230000007774 longterm Effects 0.000 description 10
- 229910052751 metal Inorganic materials 0.000 description 9
- 239000002184 metal Substances 0.000 description 9
- 238000000034 method Methods 0.000 description 7
- 239000002243 precursor Substances 0.000 description 7
- 238000004519 manufacturing process Methods 0.000 description 6
- 238000002156 mixing Methods 0.000 description 6
- 239000000843 powder Substances 0.000 description 6
- 238000003917 TEM image Methods 0.000 description 5
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 5
- 238000004458 analytical method Methods 0.000 description 5
- 239000011888 foil Substances 0.000 description 5
- 239000000203 mixture Substances 0.000 description 5
- VAYTZRYEBVHVLE-UHFFFAOYSA-N 1,3-dioxol-2-one Chemical compound O=C1OC=CO1 VAYTZRYEBVHVLE-UHFFFAOYSA-N 0.000 description 4
- VEQPNABPJHWNSG-UHFFFAOYSA-N Nickel(2+) Chemical compound [Ni+2] VEQPNABPJHWNSG-UHFFFAOYSA-N 0.000 description 4
- 239000006227 byproduct Substances 0.000 description 4
- 238000000975 co-precipitation Methods 0.000 description 4
- 230000007423 decrease Effects 0.000 description 4
- 150000002739 metals Chemical class 0.000 description 4
- 229910001453 nickel ion Inorganic materials 0.000 description 4
- 239000012466 permeate Substances 0.000 description 4
- 230000008569 process Effects 0.000 description 4
- 238000004098 selected area electron diffraction Methods 0.000 description 4
- 239000000243 solution Substances 0.000 description 4
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 3
- 238000006243 chemical reaction Methods 0.000 description 3
- 239000011247 coating layer Substances 0.000 description 3
- 238000011156 evaluation Methods 0.000 description 3
- 230000014509 gene expression Effects 0.000 description 3
- 150000002642 lithium compounds Chemical class 0.000 description 3
- 239000002002 slurry Substances 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 229910013872 LiPF Inorganic materials 0.000 description 2
- 101150058243 Lipf gene Proteins 0.000 description 2
- WMFOQBRAJBCJND-UHFFFAOYSA-M Lithium hydroxide Chemical compound [Li+].[OH-] WMFOQBRAJBCJND-UHFFFAOYSA-M 0.000 description 2
- 239000000654 additive Substances 0.000 description 2
- 238000000498 ball milling Methods 0.000 description 2
- 230000008859 change Effects 0.000 description 2
- 230000003247 decreasing effect Effects 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 239000012153 distilled water Substances 0.000 description 2
- 239000011267 electrode slurry Substances 0.000 description 2
- JBTWLSYIZRCDFO-UHFFFAOYSA-N ethyl methyl carbonate Chemical compound CCOC(=O)OC JBTWLSYIZRCDFO-UHFFFAOYSA-N 0.000 description 2
- 238000011068 loading method Methods 0.000 description 2
- 239000002105 nanoparticle Substances 0.000 description 2
- 238000005245 sintering Methods 0.000 description 2
- 239000002904 solvent Substances 0.000 description 2
- 230000001629 suppression Effects 0.000 description 2
- 230000007704 transition Effects 0.000 description 2
- NLXLAEXVIDQMFP-UHFFFAOYSA-N Ammonium chloride Substances [NH4+].[Cl-] NLXLAEXVIDQMFP-UHFFFAOYSA-N 0.000 description 1
- VHUUQVKOLVNVRT-UHFFFAOYSA-N Ammonium hydroxide Chemical compound [NH4+].[OH-] VHUUQVKOLVNVRT-UHFFFAOYSA-N 0.000 description 1
- SECXISVLQFMRJM-UHFFFAOYSA-N N-Methylpyrrolidone Chemical compound CN1CCCC1=O SECXISVLQFMRJM-UHFFFAOYSA-N 0.000 description 1
- 241000519995 Stachys sylvatica Species 0.000 description 1
- 238000000026 X-ray photoelectron spectrum Methods 0.000 description 1
- OPGVPLFFXPEXGT-UHFFFAOYSA-J [W](O)(O)(O)O.[Mn].[Co].[Ni] Chemical compound [W](O)(O)(O)O.[Mn].[Co].[Ni] OPGVPLFFXPEXGT-UHFFFAOYSA-J 0.000 description 1
- 230000001133 acceleration Effects 0.000 description 1
- 230000002776 aggregation Effects 0.000 description 1
- 238000004220 aggregation Methods 0.000 description 1
- 235000011114 ammonium hydroxide Nutrition 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 239000006229 carbon black Substances 0.000 description 1
- 150000001768 cations Chemical class 0.000 description 1
- 229910000361 cobalt sulfate Inorganic materials 0.000 description 1
- 229940044175 cobalt sulfate Drugs 0.000 description 1
- KTVIXTQDYHMGHF-UHFFFAOYSA-L cobalt(2+) sulfate Chemical compound [Co+2].[O-]S([O-])(=O)=O KTVIXTQDYHMGHF-UHFFFAOYSA-L 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- RKTYLMNFRDHKIL-UHFFFAOYSA-N copper;5,10,15,20-tetraphenylporphyrin-22,24-diide Chemical compound [Cu+2].C1=CC(C(=C2C=CC([N-]2)=C(C=2C=CC=CC=2)C=2C=CC(N=2)=C(C=2C=CC=CC=2)C2=CC=C3[N-]2)C=2C=CC=CC=2)=NC1=C3C1=CC=CC=C1 RKTYLMNFRDHKIL-UHFFFAOYSA-N 0.000 description 1
- 239000011258 core-shell material Substances 0.000 description 1
- 230000000779 depleting effect Effects 0.000 description 1
- 230000002542 deteriorative effect Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000007599 discharging Methods 0.000 description 1
- 238000001035 drying Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000004146 energy storage Methods 0.000 description 1
- 239000007789 gas Substances 0.000 description 1
- 230000001788 irregular Effects 0.000 description 1
- 229940099596 manganese sulfate Drugs 0.000 description 1
- 239000011702 manganese sulphate Substances 0.000 description 1
- 235000007079 manganese sulphate Nutrition 0.000 description 1
- SQQMAOCOWKFBNP-UHFFFAOYSA-L manganese(II) sulfate Chemical compound [Mn+2].[O-]S([O-])(=O)=O SQQMAOCOWKFBNP-UHFFFAOYSA-L 0.000 description 1
- 238000013507 mapping Methods 0.000 description 1
- 229910021645 metal ion Inorganic materials 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- LGQLOGILCSXPEA-UHFFFAOYSA-L nickel sulfate Chemical compound [Ni+2].[O-]S([O-])(=O)=O LGQLOGILCSXPEA-UHFFFAOYSA-L 0.000 description 1
- 229910000363 nickel(II) sulfate Inorganic materials 0.000 description 1
- 239000011368 organic material Substances 0.000 description 1
- 230000035515 penetration Effects 0.000 description 1
- 229920002981 polyvinylidene fluoride Polymers 0.000 description 1
- 239000013641 positive control Substances 0.000 description 1
- 238000002360 preparation method Methods 0.000 description 1
- 230000009257 reactivity Effects 0.000 description 1
- 238000012827 research and development Methods 0.000 description 1
- 238000005070 sampling Methods 0.000 description 1
- 238000001878 scanning electron micrograph Methods 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 238000004544 sputter deposition Methods 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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-
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- C01G53/00—Compounds of nickel
- C01G53/40—Nickelates
- C01G53/42—Nickelates containing alkali metals, e.g. LiNiO2
- C01G53/44—Nickelates containing alkali metals, e.g. LiNiO2 containing manganese
- C01G53/50—Nickelates containing alkali metals, e.g. LiNiO2 containing manganese of the type [MnO2]n-, e.g. Li(NixMn1-x)O2, Li(MyNixMn1-x-y)O2
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- H01M4/36—Selection of substances as active materials, active masses, active liquids
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- H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
- H01M4/381—Alkaline or alkaline earth metals elements
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- H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
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- H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
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- H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
- H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
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- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
- H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
- H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
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Abstract
The present invention relates to a positive electrode active material for a lithium secondary battery, the positive electrodeThe electrode active material has a layered structure and includes lithium, a transition metal, fluorine (F), and oxygen, wherein the layered structure includes a lithium layer composed of only lithium and a transition metal layer composed of only a transition metal including nickel, and the nickel includes Ni in terms of oxidation number 3+ And Ni 2+ And Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) Increases with increasing fluorine content.
Description
Technical Field
The present invention relates to a positive electrode active material and a lithium secondary battery including the same, and more particularly, to a positive electrode active material for a lithium secondary battery having improved cycle life and capacity characteristics by including fluorine, and a lithium secondary battery including the same.
Background
With the development of portable mobile electronic devices such as smart phones, MP3 players, and tablet personal computers, the demand for secondary batteries capable of storing electric power has increased explosively. In particular, with the advent of portable devices, medium and large-sized energy storage systems, and electric vehicles, which require high energy density, the demand for lithium secondary batteries has been increasing.
As the demand for such lithium secondary batteries increases, research and development into positive electrode active materials for lithium secondary batteries have been conducted. For example, korean patent application laid-open No. 10-2014-0119621 (korean patent application No. 10-2013-0150315) discloses such a secondary battery: which has high voltage capacity and long cycle life characteristics due to control of the kind and composition of substitute metals in precursors for preparing lithium-rich positive electrode active materials and control of the kind and amount of metals added.
Disclosure of Invention
Technical problem
An object of the present invention is to provide a fluorine-containing positive electrode active material for a lithium secondary battery having a high capacity and improved cycle characteristics while containing a large amount of nickel, and a lithium secondary battery comprising the same.
It is another object of the present invention to provide a fluorine-containing positive electrode active material for a lithium secondary battery having a layered structure of: it has enhanced structural stability due to control of the oxidation number of nickel by addition of fluorine, and shows low capacity drop even after long-term cycling.
The object to be achieved by the present invention is not limited to the above object.
Technical scheme
In order to achieve the above object, the present invention provides a fluorine-containing positive electrode active material for a lithium secondary battery and a lithium secondary battery comprising the same.
According to one embodiment, a positive electrode active material for a lithium secondary battery has a layered structure and includes lithium, a transition metal, fluorine (F), and oxygen, wherein the layered structure includes a lithium layer composed of only lithium and a transition metal layer composed of only a transition metal including nickel, wherein nickel includes Ni in terms of oxidation number 3+ And Ni 2+ And Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3 + ) Increases with increasing fluorine content.
In one embodiment, the layered structure may further include a first mixed layer and a second mixed layer each including lithium and a transition metal, wherein a content of lithium in the first mixed layer may be higher than a content of the transition metal, and a content of the transition metal in the second mixed layer may be higher than the content of lithium.
In one embodiment, the first mixed layer and the second mixed layer in the positive electrode active material may be stacked adjacent to each other and alternately and regularly repeated to form a layered structure, and the first mixed layer and the second mixed layer stacked adjacent to each other may be configured such that the transition metal of the first mixed layer and the lithium of the second mixed layer may correspond to each other.
In one embodiment, the first mixed layer and the second mixed layer stacked adjacent to each other may have an ordered structure, wherein the ordered structure may be formed such that the n1 lithium ions and the n2 transition metal ions of the first mixed layer may correspond to the n1 transition metal ions and the n2 transition metal ions, respectively, of the second mixed layer (where n1 and n2 are the same or different natural numbers), and the unit cell formed of the ordered structure may include a long-range ordered lattice having an increased a-axis lattice constant.
In one embodiment, the lattice formed of the first mixed layer and the second mixed layer stacked adjacent to each other may include a superlattice whose a-axis is twice as long as the lattice formed of the lithium layer and the transition metal layer.
In one embodiment, a peak (I) of a (003) plane in an X-ray diffraction spectrum of the positive electrode active material obtained by XRD analysis using CuK α radiation after the electrochemical reaction (003) ) Peak value (I) of and (104) plane (104) ) Ratio (I) of (003) /I (104) ) The reduction in (c) may be less than 1%.
In one embodiment, the peak (I) of the (003) plane (103) ) Peak value (I) of and (104) plane (104) ) Ratio of (I) (003) /I (104) ) And may be 1.71 or less.
In one embodiment, Ni2P has a binding energy of 850eV to 860eV in X-ray photoelectron Spectroscopy (XPS) spectra of Ni2P obtained by XPS 2+ May be greater than Ni 3+ And Ni 2+ And Ni 3+ Ratio of (Ni) 2 + /Ni 3+ ) May be 49% to 130%.
In one embodiment, Ni is at a binding energy of 850eV to 860eV 2+ Peak area of (2) and Ni 3+ The ratio of the peak areas of (a) may be 0.49:1 to 1.3: 1.
In one embodiment, the positive electrode active material may include secondary particles composed of a group of a plurality of primary particles, and at least one of the primary particles may include a fluorine-containing grain coating at grain boundaries between the primary particles.
In one embodiment, the positive electrode active material may be represented by the following formula 1:
[ formula 1]
Li 1-x M 1-y [Li x M y ]O 2-z F z
Wherein x + y is 1; z is more than or equal to 0.005 and less than or equal to 0.02; and M is any one of: ni; ni and Co; ni and Mn; ni, Co and Mn; ni and Al; ni, Co and Al; ni, Mn and Al; and Ni, Co, Mn and Al.
In one embodiment, Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) May be decreased as the content of nickel in formula 1 is increased.
In one embodiment, the layered structure may further include a first mixed layer and a second mixed layer each containing lithium and a transition metal; the content of lithium in the first mixed layer is higher than that of the transition metal, and the content of the transition metal in the second mixed layer is higher than that of lithium; the first mixed layer and the second mixed layer in the positive electrode active material are stacked adjacent to each other and alternately and regularly repeated to form a layered structure; and the electron diffraction pattern of the [010] or [100] crystallographic band axis (zone axis) of the layered structure can show: a first diffraction spot group in which one or more diffraction spots having a first intensity and corresponding to a crystal lattice formed by lithium layers and transition metal layers stacked adjacent to each other are aligned in one direction; and a second diffraction spot group in which one or more diffraction spots having a second intensity lower than the first intensity of the diffraction spots of the first diffraction group and corresponding to a crystal lattice formed by the first mixed layer and the second mixed layer stacked adjacent to each other are aligned in one direction.
In one embodiment, the first and second diffraction spot groups may be alternately and regularly arranged with each other, and the first and second diffraction spot groups may be spaced apart from each other.
In one embodiment, the layered structure may further include a first mixed layer and a second mixed layer each including lithium and a transition metal, wherein a content of lithium in the first mixed layer may be higher than a content of the transition metal, a content of the transition metal in the second mixed layer may be higher than a content of lithium, lithium and the transition metal in the first mixed layer may be alternately arranged, transition metal and lithium in the second mixed layer may be alternately arranged, and a lattice formed by the first mixed layer and the second mixed layer stacked adjacent to each other may include a superlattice.
In one embodiment, the transition metal includes any one or more of nickel (Ni), manganese (Mn), and cobalt (Co), wherein the oxidation number of nickel is 2+ and 3+, and the oxidation number of manganese is 3+ or 4+, and the oxidation number of cobalt may be 3 +.
In one embodiment, the primary particles may include rod-shaped particles formed in a plate-like shape having a long axis and a short axis in cross section, and the rod-shaped particles may be oriented such that the long axis thereof faces the central portion of the secondary particles.
In one embodiment, the transition metal may include any one or more of nickel (Ni), manganese (Mn), and cobalt (Co), wherein at least one of the transition metals may have a concentration gradient from the center of the secondary particle toward the surface thereof in at least a portion of the secondary particle.
In one embodiment, nickel (Ni) may be included in an amount of 70 mol% or more.
According to another aspect of the present invention, one embodiment of the present invention includes a lithium secondary battery including: a positive electrode containing the above positive electrode active material for a lithium secondary battery; a negative electrode facing the positive electrode and composed of graphite or lithium metal; a separator interposed between the positive electrode and the negative electrode; and an electrolyte solution or a solid electrolyte containing a lithium salt.
Advantageous effects
According to the present invention as described above, it is possible to provide a fluorine-containing positive electrode active material for a lithium secondary battery, which has a high content of nickel and has improved cycle-life characteristics due to suppression of side reactions between an electrolyte and the positive electrode active material while maintaining a high discharge capacity, and a lithium secondary battery including the same.
Further, according to the present invention, it is possible to provide a fluorine-containing positive electrode active material for a lithium secondary battery, which has a layered structure and a new crystal structure maintained during long-term cycling while having a high content of nickel (Ni-rich), and thus has improved stability and reliability, and a lithium secondary battery including the same.
Drawings
Fig. 1 schematically shows a secondary particle according to an embodiment of the present invention.
Fig. 2 is a schematic sectional view of a positive electrode active material according to an embodiment of the present invention.
Fig. 3 schematically shows the movement of lithium ions in the positive electrode active material according to an embodiment of the present invention.
Fig. 4 schematically shows images (a and b) of the positive electrode active material before and after addition of fluorine and heat treatment and an ordered structure (c) of the positive electrode active material formed by addition of fluorine according to example 1 of the present invention.
Fig. 5 depicts graphs showing electrochemical capacity (a) and cycle-life characteristics (b) of coin cells employing the positive electrode active materials according to examples 1 to 3 of the present invention and comparative example 1.
Fig. 6 is a graph showing the electrical resistance of coin cells employing the positive electrode active materials according to comparative example 1 and example 2 of the present invention.
Fig. 7 depicts a graph showing cycle characteristics and resistance versus SOC of a full battery employing the positive electrode active materials according to comparative example 1 and example 2 of the present invention.
Fig. 8 shows a TEM image showing the ordered structure of the positive electrode active material according to example 2 after 2000 cycles.
Fig. 9 shows a TEM image showing the ordered structure of the positive electrode active material according to example 2 after 5000 cycles.
Fig. 10 shows the results of XRD analysis performed to check the c-axis length and the a-axis length after the cycles of the positive electrode active materials according to comparative example 1 and example 2.
Fig. 11 depicts a graph (a) showing peaks on the (003) plane and peaks on the (104) plane and a graph (b) showing a ratio between the peaks in the positive electrode active material according to example 2.
Fig. 12 shows XRD patterns according to example 2, example 3, comparative example 1 and comparative example 2.
Fig. 13 shows XPS charts according to example 2, example 3, comparative example 1, and comparative example 2.
Fig. 14 shows selected-area electron diffraction (SAED) patterns of [010] ribbon axes according to comparative example 1 and example 2.
Fig. 15 shows a high-angle annular dark field (HAADF) image of the [010] ribbon axis according to comparative example 1 and example 2.
Detailed Description
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the technical spirit of the present invention is not limited to the embodiments disclosed herein, and may be embodied in other forms. Rather, the embodiments disclosed herein are provided so that this disclosure will be thorough and complete, and will fully convey the spirit of the invention to those skilled in the art.
Throughout this specification, when any component is referred to as being "on" another component, it refers not only to the case where the component is directly on the other component, but also to the case where a third component exists between the two components. In addition, in the drawings, the thickness of the film and the region is exaggerated for effectively describing the technical contents.
Also, terms such as first, second, third, etc. are used in different embodiments of the present specification to describe different components, but the components should not be limited by the terms. These terms are only used to distinguish one component from another component. Thus, a component that is referred to as a first component in any one embodiment may also be referred to as a second component in other embodiments. Each of the embodiments described and illustrated herein also includes additional embodiments thereof. Further, as used herein, the term "and/or" includes at least one of the associated listed items.
In this specification, the singular expressions include plural expressions unless the context clearly dictates otherwise. The terms "comprising," "including," "having," and the like, are intended to indicate the presence of the stated features, numbers, steps, components, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.
In the following description, a detailed description of known functions or configurations related thereto will be omitted when it may obscure the subject matter of the present invention.
Further, in the present specification, the expression "the proportion of the first crystal structure is higher than the proportion of the second crystal structure in a specific portion" means that the specific portion includes both the first crystal structure and the second crystal structure, and means not only that the proportion of the first crystal structure is higher than the proportion of the second crystal structure in the specific portion, but also that the specific portion has only the first crystal structure.
Further, in the present specification, the crystal system may be composed of the following seven crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal or rhombohedral, hexagonal, and cubic systems.
Further, in the present application, "mol%" is interpreted to mean the content of any metal in the positive electrode active material or the positive electrode active material precursor, based on 100% of the total of the metal other than lithium and oxygen in the positive electrode active material or the positive electrode active material precursor.
Fig. 1 schematically shows a secondary particle according to an embodiment of the present invention. Fig. 2 is a schematic sectional view of a positive electrode active material according to an embodiment of the present invention. Fig. 3 schematically shows movement of lithium ions in a positive electrode active material according to an embodiment of the present invention.
Referring to fig. 1 to 3, a positive electrode active material according to an embodiment of the present invention may include secondary particles 100 having an approximately spherical shape. The secondary particle 100 may be divided into the central portion 10 and the surface portion 20, and may be formed by aggregation of a plurality of primary particles 30. The primary particles 30 may form a layered structure.
In the secondary particle 100, the primary particle 30 constituting the central portion 10 and the primary particle 30 constituting the surface portion 20 may have the same or different sizes, and the primary particle 30 constituting the surface portion 20 may be provided in a shape having a major axis and a minor axis, and at least a part of the primary particle 30 constituting the surface portion 20 may be oriented toward the central portion 10 of the secondary particle 100. At least a part of the primary particles 30 constituting the surface portion 20 may be arranged in a radial form.
The secondary particles 100 may be composed of one or more metals including lithium, and at least a portion of the one or more metals may be disposed to have a concentration gradient in the central portion 10 and the surface portion 20. Alternatively, in the secondary particle 100, a gap may be formed between the central portion 10 and the surface portion 20, or the crystal form between the central portion 10 and the surface portion 20 may be different, or an image difference between the central portion 10 and the surface portion 20 may be created by manufacturing the central portion 10 and then surrounding the surface portion 20 around the central portion 10. Thus, the secondary particles may be provided in a core-shell form.
The primary particles 30 may extend radially from a region in the secondary particles towards the surface portion 20 of the secondary particles. This one region in the secondary particle may be the central part 10 of the secondary particle. In other words, at least a part of the primary particles 30 may include rod-shaped particles formed into a plate-like shape. The rod-shaped particles may be oriented such that their long axes face the central portion of the secondary particles.
Between the primary particles 30, that is, between the primary particles 30 extending in the direction D from the central portion 10 of the secondary particle toward the surface portion 20 thereof, a moving path of metal ions (e.g., lithium ions) and the electrolyte may be provided. Therefore, the positive electrode active material according to one embodiment of the present invention can improve the charge and discharge efficiency of the secondary battery.
According to one embodiment, the length of the primary particle 30 relatively adjacent to the surface 20 of the secondary particle may be longer than the length of the primary particle 30 relatively adjacent to the central portion 10 of the secondary particle in a direction from the center 10 of the secondary particle towards the surface 20 of the secondary particle. In other words, the length of the primary particles 30 may increase towards the surface 20 in at least a part of the secondary particles extending from the central portion 10 of the secondary particles to the surface 20.
Fig. 4 schematically shows images (a and b) of the positive electrode active material before and after addition of fluorine and heat treatment and an ordered structure (c) of the positive electrode active material formed by addition of fluorine according to an embodiment of the present invention.
One embodiment of the present invention includes a positive electrode active material for a lithium secondary battery having a layered structure and including lithium, a transition metal, fluorine (F), and oxygen, wherein the layered structure includes a lithium layer consisting of only lithium and a transition metal layer consisting of only a transition metal including nickel. In terms of oxidation number, the nickel may include Ni 3+ And Ni 2+ And Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) May increase with increasing fluorine content. The fluorine-containing positive electrode active material for a lithium secondary battery may be prepared by: a metal composite hydroxide is prepared using a transition metal containing nickel, the metal composite hydroxide is mixed with a lithium compound to prepare a sintered body, and the sintered body is mixed with a fluorine compound and then subjected to a heat treatment. For example, in the positive electrode active material, nickel (Ni) may be included in an amount of 70 mol% or more.
In general, in a nickel-rich layered positive electrode active material having a nickel content of 70 mol% or more, high capacity can be achieved by nickel, but there is a problem in that: side reactions occur with the electrolyte on the surface of the positive electrode active material during cycling, and the resulting by-products degrade cycle life characteristics.
Such side reactions cause unnecessary consumption of electrolyte, thereby depleting the electrolyte in the lithium secondary battery, and by-products composed of organic materials are accumulated on the electrode surface, thereby reducing the coulombic efficiency of the lithium secondary battery and causing instability. In addition, during the charge and discharge processes, the by-products may cause deterioration of the crystal structure of the particles of the positive electrode active material, thereby deteriorating the cycle-life characteristics. In particular, in the nickel-rich positive electrode active material, the by-product causes suppression of the phase transition from H2 to H3, resulting in rapid deterioration of cycle life characteristics.
On the other hand, in the case of the positive electrode active material according to this embodiment, the oxidation number of nickel can be controlled by adding fluorine and heat treatment. In particular, Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) May increase with increasing fluorine content.
The positive electrode active material may have a layered structure in which, in addition to a lithium layer composed only of lithium and a transition metal layer made only of a transition metal, the layered structure may include a first mixed layer and a second mixed layer each containing lithium and a transition metal. The content of lithium in the first mixed layer may be higher than that of the transition metal, the content of the transition metal in the second mixed layer may be higher than that of lithium, and nickel contained in the first mixed layer may be nickel (Ni) having a specific oxidation number of +3 3+ ) Larger amounts include nickel (Ni) with an oxidation number of +2 2+ ) And specifically, may be composed of only nickel (Ni) having an oxidation number of +2 2+ ) And (4) forming.
The positive electrode active material may include secondary particles composed of a group of a plurality of primary particles, and at least one of the primary particles may include a fluorine-containing grain coating layer at grain boundaries between the primary particles. The metal composite hydroxide containing a transition metal is mixed with a lithium compound and then prepared into a sintered body, and the prepared sintered body is mixed with a fluorine compound using ball milling or the like. Thus, the surface of the sintered body is coated with the fluorine compound (F) (see (a) of fig. 4). Then, when the sintered body is heat-treated at 300 to 700 ℃, specifically about 400 ℃, the fluorine compound applied to the surface of the sintered body can be uniformly diffused into the sintered body, and can be formed as a grain coating at the grain boundary between the primary particles.
After mixing the sintered body with the fluorine compound, heat treatment may be performed at 300 to 700 ℃. If the heat treatment is performed at less than 300 ℃, the fluorine compound may be difficult to uniformly diffuse into the sintered body, and if the heat treatment is performed at more than 700 ℃, the electrochemical performance of the positive electrode active material may be deteriorated.
The primary particles having the grain coating formed thereon can be prevented from undergoing long-term cyclingStopping side reactions with the electrolyte at the boundaries between primary particles, thereby maintaining the lithium ion movement path (Li) + Path) so that the movement of lithium ions can be effectively achieved. In addition, these primary particles can prevent side reactions with the electrolyte during charge/discharge and prevent the layered structure from being transformed into a cubic crystal structure during charge/discharge. For example, the primary particles having the fluorine-containing grain coating layer formed thereon may include rod-shaped particles.
In the positive electrode active material in which a grain coating layer is formed on at least a part of the primary particles by adding fluorine and heat treatment, Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) May be increased. Li + Has an ionic radius of
Ni 2+ Has an ionic radius of
And Ni 3+ Has an ionic radius of
Or larger. Due to Ni 2+ Ionic radius of (2) and Li + Is substantially similar to Ni 3+ In contrast, Ni 2+ Probabilistically favour Li + Site exchange was performed. Further, in this embodiment, by the control of the addition of fluorine and the heat treatment, the sites of lithium ions and nickel ions can be regularly moved toward each other, thereby providing a positive electrode active material having a value expanded to a new lattice constant. In the positive electrode active material, Ni can be increased by adding fluorine and heat treatment 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) And the first mixed layer and the second mixed layer may be formed by site exchange between lithium in the initial lithium layer and nickel in the transition metal layer.
For example, the first mixed layer may pass through the regular position between lithium in a layer composed only of lithium and nickel in an adjacent layer composed only of a transition metalDot exchange is performed, and in this case, the nickel transferred to the first mixed layer may be Ni 2+ 。Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) Can be controlled by adding fluorine and heat treatment when Ni is 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) Above a predetermined level, the first mixed layer and the second mixed layer adjacent to each other may pass Li + And Ni 2+ Or Ni 3+ Is not an irregular mixture of lithium and nickel (cation mixture). In particular, Li + And Ni 2+ Can have more regular site exchanges than Li + And Ni 3+ Regular site exchange between, more specifically, Li + Can be reacted with Ni 2+ Site exchange was performed. The first mixed layer may be formed by providing Ni at a lithium site in a layer composed of lithium only 2+ And the layer formed, the second mixed layer may be formed by providing Li at a nickel site in a layer composed of only a transition metal containing nickel + Thereby forming the composite material.
In the positive electrode active material, the first mixed layer and the second mixed layer may be stacked adjacent to each other and alternately and regularly repeated to form a layered structure, and the first mixed layer and the second mixed layer stacked adjacent to each other may be disposed such that the transition metal of the first mixed layer and the lithium of the second mixed layer correspond to each other. Specifically, the first mixed layer and the second mixed layer stacked adjacent to each other may be provided such that the transition metal of the first mixed layer and the lithium of the second mixed layer correspond to each other.
More specifically, the first mixed layer and the second mixed layer stacked adjacent to each other may have a cation-ordered structure, wherein the cation-ordered structure may be formed such that n1 lithium ions and n2 transition metal ions of the first mixed layer may correspond to n1 transition metal ions and n2 transition metal ions, respectively, of the second mixed layer (where n1 and n2 are the same or different natural numbers).
The unit cell formed by the ordered structure may include a long-range ordered lattice having an increased a-axis lattice constant. The lattice formed of the first mixed layer and the second mixed layer stacked adjacent to each other may include a superlattice whose a-axis is twice as long as that of the lattice formed of the lithium layer and the transition metal layer.
In the positive electrode active material according to this embodiment, the fluorine-containing grain coating and the ordered structure formed by adding fluorine and heat treatment can suppress side reactions at the interface between the positive electrode active material and the electrolyte.
In addition, the grain coating can further improve electrochemical performance during charge/discharge by making the interface between primary particles more stable. In addition, the ordered structure may maximize lattice stability so that crystallinity and lattice structure may be stably maintained even in repeated charge and discharge cycles.
The positive electrode active material may be represented by formula 1 below:
[ formula 1]
Li 1-x M 1-y [Li x M y ]O 2-z F z
Wherein x + y is 1; z is more than or equal to 0.005 and less than or equal to 0.02; m is Ni; ni and Co; ni and Mn; and any one of Ni, Co and Mn.
In formula 1, Li 1-x May represent a lithium layer consisting only of lithium, M 1-y Can represent a transition metal layer composed of only a transition metal, [ Li ] x M y ]The first mixed layer and the second mixed layer may be represented.
In formula 1, Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) May decrease with increasing nickel content. Furthermore, Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) May increase with fluorine content.
The transition metal may include at least one of nickel (Ni), manganese (Mn), and cobalt (Co), wherein the oxidation number of nickel may be 2+ and 3+, the oxidation number of manganese may be 3+ or 4+, and the oxidation number of cobalt may be 3 +.
The transition metal includes any one or more of nickel (Ni), manganese (Mn), and cobalt (Co), wherein at least one of the transition metals may have a concentration gradient from the center of the secondary particle toward the surface thereof in at least a portion of the secondary particle.
Specifically, at least one of the transition metals may be provided with a concentration gradient from a central portion to a surface portion of the secondary particle, or may be provided with a concentration gradient from the center of the secondary particle to a predetermined portion of the secondary particle and set at a constant concentration from the predetermined portion to the surface of the secondary particle, or may be set at a constant concentration from the center of the secondary particle to a predetermined portion thereof and set with a concentration gradient from a predetermined portion to the surface of the secondary particle.
The positive electrode active material may include secondary particles composed of a group of a plurality of primary particles, wherein the secondary particles may include microcracks extending from an outermost surface of the secondary particles to an inside of the secondary particles, and the microcracks are channels through which the electrolyte permeates the secondary particles. In a secondary battery using the positive electrode active material, the secondary particles may maintain 80% or more of an initial specific capacity after 5000 charge/discharge cycles at a discharge rate of 1C. That is, the positive electrode active material according to this embodiment can maintain 80% or more of the initial specific capacity after a long period of cycles, for example, 2000 cycles or 5000 cycles, in which case, even if the electrolyte permeates the inside of the positive electrode active material, no side reaction occurs due to high stability (crystallinity) of the grain coating on the primary particles constituting the positive electrode active material. This is because, although the positive electrode active material according to this embodiment may generate microcracks during long-term cycling, even when the electrolyte permeates the secondary particles through the microcracks, problems such as the secondary particles collapsing as a whole do not occur.
The positive electrode active material according to one embodiment of the present invention may be a secondary particle composed of a group of a plurality of primary particles, wherein the secondary particle may have an approximately spherical shape. The primary particles are oriented from a central portion of the secondary particles towards a surface portion thereof, and the shape of the primary particles in the central portion may be different from the shape of the primary particles in the surface portion.
After a plurality of cycles, microcracks may form in spaces between primary particles in the secondary particles of the positive electrode active material.
Generally, in a positive electrode active material in which the cross-sectional area of microcracks is 20% or more, an electrolyte permeates the center of secondary particles and causes a side reaction during charge and discharge, so that the structure of primary particles rapidly collapses, so that the capacity thereof decreases during cycling.
On the other hand, when the sectional area of the microcracks in the positive electrode active material according to this embodiment is 20% or more, in a secondary battery using the positive electrode active material, the positive electrode active material can maintain 80% or more of the initial specific capacity after 5000 charge/discharge cycles at a discharge rate of 1C, and can exhibit excellent cycle characteristics. That is, the positive electrode active material according to the embodiment of the invention can prevent penetration of an electrolyte due to a grain coating and an ordered structure even when microcracks are formed, and can exhibit excellent characteristics without decreasing capacity due to an ordered structure having low reactivity with an electrolyte due to high crystallinity (stability) even when an electrolyte penetrates the positive electrode active material.
According to another aspect of the present invention, one embodiment of the present invention includes a lithium secondary battery including: a positive electrode containing the above positive electrode active material; a negative electrode facing the positive electrode and composed of graphite or lithium metal; a separator interposed between the positive electrode and the negative electrode; and an electrolyte solution or a solid electrolyte containing a lithium salt.
According to still another aspect of the present invention, one embodiment of the present invention may include a method for preparing a positive electrode active material for a lithium secondary battery comprising an ordered structure substituted with lithium and a transition metal, the method comprising the steps of: preparing a metal composite hydroxide from a transition metal by a coprecipitation method; preparing a sintered body by sintering a metal composite hydroxide with a lithium compound; the sintered body is mixed with a fluorine compound and then heat-treated at 300 to 700 ℃.
The transition metal may include nickel (Ni).
In the step of preparing the sintered body, the metal composite hydroxide may be presintered at a certain temperature, and then sintered at a temperature higher than the presintering temperature.
Subsequently, the sintered body may be reacted with a fluorine compound such as ammonium fluoride (NH) 4 F) Mixing and then heat treating at 300 ℃ to 700 ℃ to form a fluorine-containing grain coating on at least a portion of the surface of the sintered body. The positive electrode active material may include secondary particles that are composed of groups of primary particles and have an approximately spherical shape, and a grain coating may be formed on at least one of the primary particles at grain boundaries between the primary particles.
The sintered body and the fluorine compound may be mixed together at room temperature using a ball mill before the heat treatment. By mixing the sintered body and the fluorine compound under pressure using ball milling, the fluorine compound can be diffused into the sintered body after the heat treatment.
Hereinafter, examples of the present invention and comparative examples will be described. However, the following examples are only preferred embodiments of the present invention, and the scope of the present invention is not limited by the following examples.
1. Preparation of positive electrode active material
Example 1 (F0.5-concentration gradient type NCM80)
10 liters of distilled water was placed in a coprecipitation reactor (volume: 40 liters) and then N was added 2 The gas was supplied to the reactor at a rate of 6 liters/min and stirred at 350rpm while the reactor temperature was maintained at 40 ℃. An aqueous nickel sulfate solution (NiSO) is added in an amount such that the molar ratio between nickel (Ni), cobalt (Co) and manganese (Mn) is 80:5:15 4 ·6H 2 O, Samchun Chemicals), aqueous cobalt sulfate (CoSO) 4 ·7H 2 O, Samchun Chemicals) and aqueous manganese sulfate (MnSO) 4 ·H 2 O, Samchun Chemicals) were mixed together to prepare a 2M metal solution. The prepared 2M metal solution and 16M ammonia solution (NH) 4 OH, JUNSEI) were continuously introduced into the reactor at a rate of 0.561 liter/hr and 0.08 liter/hr, respectively. During the coprecipitation reaction, the pH in the reactor was checked and adjusted by adding NaOH (aq.) solution to the reaction mixtureThe pH was maintained at 11.4 in the reactor.
The co-precipitation reaction is carried out in a reactor during which first the nanoparticle hydroxide is formed and then the nickel-cobalt-manganese-tungsten hydroxide slowly accumulates on the surface of the nanoparticle hydroxide to form a micron-sized precursor. Next, the prepared precursor was washed several times with distilled water, filtered through a filter, and dried in a drying oven at 110 ℃ for 12 hours to obtain [ Ni ] 0.80 Co 0.05 Mn 0.15 ](OH) 2 A metal composite hydroxide.
To obtain [ Ni ] 0.80 Co 0.05 Mn 0.15 ](OH) 2 Metal composite hydroxide and LiOH H 2 O is uniformly mixed together so that the molar ratio of Li (Ni + Co + Mn) is 1.01: 1. Then, the mixture was sintered at 770 ℃ for 10 hours to obtain Li [ Ni ] 0.80 Co 0.05 Mn 0.15 ]O 2 The sintered body of (1). Then, the sintered body was mixed with ammonium fluoride (NH) 4 F) Mixed and then heat-treated at 400 ℃ to obtain a positive electrode active material powder containing 0.5 mol% of fluorine.
Example 2 (F1-concentration gradient type NCM80)
A positive electrode active material powder was obtained in the same manner as in example 1, except that the sintered body was mixed with ammonium fluoride (NH) 4 F) Mixed and then heat treated at 400 c so that it contains 1 mol% fluorine.
Example 3 (F2-concentration gradient type NCM80)
A positive electrode active material powder was obtained in the same manner as in example 1, except that the sintered body was mixed with ammonium fluoride (NH) 4 F) Mixed and then heat treated at 400 ℃ so that it contains 2 mol% of fluorine.
Comparative example 1 (concentration gradient type NCM80)
A sintered body of Li [ Ni ] was obtained in the same manner as in example 1 0.80 Co 0.05 Mn 0.15 ]O 2 Positive electrode active material powder is constituted so as to differ in that the sintered body and ammonium fluoride (NH) are omitted 4 F) Mixing and heat treating the mixture.
Comparative example 2 (F10-concentration gradient type NCM80)
A positive electrode active material powder was obtained in the same manner as in example 1, except that the sintered body was mixed with ammonium fluoride (NH) 4 F) Mixed and then heat treated at 400 c so that it contains 10 mol% fluorine.
Table 1 below shows the contents of examples 1 to 3, comparative example 1 and comparative example 2.
[ Table 1]
| Distinguishing between | Metal composite hydroxide | Content of F |
| Example 1 | Gradient NCM 80:5:15 | 0.5mol％ |
| Example 2 | Gradient NCM 80:5:15 | 1mol％ |
| Example 3 | Gradient NCM 80:5:15 | 2mol％ |
| Comparative example 1 | Gradient NCM 80:5:15 | 0 |
| Comparative example 2 | Gradient NCM 80:5:15 | 10mol％ |
2. Half cell and full cell were manufactured using examples and comparative examples
The positive electrode active materials according to the above examples and comparative examples were used to manufacture half cells and full cells.
To manufacture half cells and full cells, the powder type positive electrode active material prepared according to each of examples and comparative examples, poly (vinylidene fluoride), and carbon black were added to N-methylpyrrolidone (0.4g per g of positive electrode active material) in a weight ratio of 90: 4.5: 5.5, and then uniformly mixed together, thereby preparing a positive electrode slurry. An aluminum foil was coated with each of the prepared positive electrode slurries, rolled, and then dried under vacuum, thereby preparing a positive electrode.
In order to manufacture a half cell using the prepared positive electrode active materials, aluminum foil was coated with each of the prepared positive electrode active material slurries so that the loading level of the positive electrode active material was 5mg/cm 2 (means when sampling 1cm from the aluminum foil coated with the positive electrode active material 2 The weight of only the positive electrode active material in the positive electrode was 5mg), thereby preparing a positive electrode. The electrolyte used was prepared by mixing 2% by weight of Vinylene Carbonate (VC) and 1.2mol/L of lithium salt LiPF as additives 6 Uniformly dissolved in ethyl methyl carbonate (EC: EMC ═ 3:7v/v) as a solvent. As a half cell, a 2032 button-type half cell (hereinafter referred to as a button cell) using Li metal as a negative electrode was manufactured.
In order to manufacture a full cell using the prepared positive electrode active materials, aluminum foil was coated with each of the prepared positive electrode active material slurries such that the loading level of the positive electrode active material was 8.5mg/cm 2 Thereby preparing a positive electrode. Furthermore, the prepared graphite slurry was used at a rate of 6.5mg/cm 2 Loaded water ofAn aluminum foil was coated flat, rolled, and then dried under vacuum, thereby preparing a negative electrode. The electrolyte used was prepared by mixing 2% by weight of Vinylene Carbonate (VC) and 1.2mol/L of lithium salt LiPF as additives 6 Uniformly dissolved in ethyl methyl carbonate (EC: EMC ═ 3:7v/v) as a solvent. Each of the positive electrode, separator (Celgard, model 2320) and negative electrode was stacked in a pouch-type battery case and sealed together with the prepared electrolyte, thereby manufacturing a pouch-type full cell.
3. Evaluation of examples and comparative examples
(1) Evaluation of capacity and cycle characteristics Using half cells
Each of the manufactured half-cells was subjected to 100-cycle charge/discharge tests by charging to 4.3V and discharging to 2.7V at 30 ℃ at a constant current of 0.5C (1C: 180mA/g), and the recovered capacity (hereinafter referred to as 2.7V to 4.3V) was measured.
(2) Evaluation of capacity and cycle characteristics Using full cells
The manufactured full cell was cycled between 3.0V (discharge voltage) and 4.2V (charge voltage) at 25 ℃ at a constant current of 1C, and the capacity and the recovered capacity were measured.
(3) The microstructures of the metal composite hydroxide (precursor) and the positive electrode active material were analyzed using SEM, TEM, XRD, and XPS
For the positive electrode active materials according to examples and comparative examples and the metal composite hydroxide (precursor) before pre-sintering used to form the positive electrode active material, the microstructure, crystal structure, and the like were analyzed by SEM (Nova Nano SEM 450, FEI). The positive electrode active material particles were cut, and a cross section of the positive electrode active material was imaged using a Transmission Electron Microscope (TEM).
X-ray diffraction (XRD) analysis was performed using CuK α radiation as an X-ray diffraction light source. The analysis was performed at a scan rate of 1 °/minute over a range of 2 θ values from 15 ° to 70 °, and at a scan rate of 0.2 °/minute over a range of 2 θ values from 19 ° to 23 °.
XPS analysis of the positive electrode active material was performed using a Quantum 2000(Physical Electronics) system. XPS analysis was performed using Quantum 2000(Physical electronics. Inc.) (acceleration voltage: 0.5keV to 15keV, 300W, energy resolution: about 1.0eV, minimum analysis area: 10 microns, sputtering rate: 0.1 nm/min).
Fig. 4 shows a mapping image of the positive electrode active material according to example 1 before and after fluorine coating and heat treatment, and a HAADF-TEM image of an ordered structure.
Fig. 5 depicts graphs showing electrochemical capacity (a) and cycle-life characteristics (b) of coin cells employing the positive electrode active materials according to examples 1 to 3 of the present invention and comparative example 1. Table 2 below shows the numerical values of the capacity and cycle life characteristics of the examples and comparative examples according to the present invention.
[ Table 2]
Referring to fig. 5 and table 2 above, it can be confirmed that the entire capacity is maintained even if fluorine is added. Further, it can be confirmed that examples 1 to 3 exhibited superior cycle life characteristics as compared to comparative example 1 in which fluorine was not added. For example, in the case of a half cell, the variation in 0.1C capacity and 0.5C capacity at a charge and discharge voltage of 2.7V to 4.3V at room temperature (25 ℃ to 30 ℃) should be reduced by 5% or less, relative to comparative example 1 to which no fluorine is added. On the other hand, it was confirmed that in the case of comparative example 2, the capacity characteristics were deteriorated due to an excessively high fluorine content. In the case of examples 1 to 3, the ordered structure controlled by adding fluorine and grain coating improves structural stability and thus long-term cycle characteristics. On the other hand, in the case of comparative example 2 in which the fluorine content was excessively high, the thickness of the grain coating formed at the grain boundary of the primary particles became excessively large, so that the channel space for lithium ion movement may be reduced, thereby reducing the efficiency of lithium ion movement.
Fig. 6 is a graph showing the electrical resistance of coin cells employing the positive electrode active materials according to comparative example 1 and example 2 of the present invention. Table 3 below shows resistance values depending on the number of cycles of comparative example 1 and example 1 of the present invention.
[ Table 3]
| Distinguishing | Content of |
25 |
50 |
75 |
100 cycles |
| Comparative example 1 | 0mol％ | 12.78Ω | 17.83 | 21.4Ω | 26.7Ω |
| Example 2 | 1mol％ | 8.41Ω | 8.64Ω | 9.15Ω | 9.92Ω |
Referring to fig. 6 and table 3, it can be confirmed that the resistance value increases as the number of cycles increases in the case of comparative example 1, while the resistance value hardly changes even when the number of cycles increases in the case of example 2. That is, it was confirmed that, in the case of comparative example 1, the positive electrode active material and the electrolyte underwent side reactions during the cycle to generate resistance at the interface therebetween, whereas in the case of example 2, the side reactions hardly occurred during the cycle. Therefore, it can be confirmed that in the case of example 2, the electrochemical characteristics are kept constant with almost no change during the cycle process, as compared with the case of example 1.
Fig. 7 depicts a graph showing cycle characteristics and resistance versus SOC of full batteries using the positive electrode active materials according to comparative example 1 and example 2 of the present invention. Fig. 8 shows a TEM image of the ordered structure of the positive electrode active material according to example 2 after 2000 cycles. Fig. 9 shows a TEM image showing the ordered structure of the positive electrode active material according to example 2 after 5000 cycles.
Table 4 shows the resistance values depending on the SOC of comparative example 1 and comparative example 2.
[ Table 4]
Referring to fig. 7 and table 4, the results of 2000 charge/discharge cycles at 1C using the full cells of comparative example 1 and example 2 are shown, and it can be determined that, in the case of comparative example 1, as the number of cycles increases, the capacity decreases and the resistance increases. On the other hand, it was confirmed that in the case of example 2, the capacity hardly changed even when the number of cycles was increased, and the capacity remained constant even after 5000 cycles without significant change in the resistance. It was confirmed that in the case of example 2, side reactions with the electrolyte did not occur due to the ordered structure and the grain coating even during long-term cycling, and thus example 1 could exhibit stable electrochemical characteristics without structural collapse, whereas in the case of comparative example 1, changes in crystal structure, such as separation of lithium ions from the lithium layer, and partial layer structure to cubic crystal structure, did occur during the cycling process, and thus electrochemical characteristics were deteriorated. Referring to the SEM image in fig. 7, comparing (c) showing the cross section of the particle of 2000 cycles of comparative example 1, (d) showing the cross section of the particle of 2000 cycles of example 2, and (e) showing the cross section of 5000 cycles of example 2, it can be determined that the microcracks in the particle itself are almost similar and the sectional area of the microcracks is about 20% or more of the total area of the secondary particles. That is, it was confirmed that, even when the cross-sectional area of the microcracks was 20% or more, the structural stability of the positive electrode active material according to the embodiment of the invention was maintained due to the grain coating formed by the addition of fluorine and the heat treatment and the ordered structure controlled by the addition of fluorine and the heat treatment, the positive electrode active material did not show a capacity decrease and an increase in resistance during the cycle process, and the positive electrode active material had excellent electrochemical characteristics.
Fig. 8 shows an image of the positive electrode active material of example 2 after 2000 cycles. Fig. 9 shows an image of the positive electrode active material of example 2 after 5000 cycles. From fig. 8 and 9, it can be determined that the ordered structure is maintained.
Fig. 8 and 9 show electron diffraction patterns of [010] or [100] crystallographic band axes. The electron diffraction pattern shows: a first diffraction spot group G1 in which one or more diffraction spots having a first intensity and corresponding to a crystal lattice formed of a lithium layer and a transition metal layer stacked adjacent to each other are aligned in one direction; and a second diffraction spot group G2 in which one or more diffraction spots having a second intensity relatively lower than the first intensity of the diffraction spots of the first diffraction group G1 and corresponding to a crystal lattice formed by the first mixed layer and the second mixed layer stacked adjacent to each other are aligned in one direction. The first and second sets of diffraction spots indicate an ordered structure.
Fig. 10 shows the results of XRD analysis performed to examine the c-axis length and the a-axis length of the positive electrode active materials according to comparative example 1 and example 2 after cycling. Table 5 below shows the c-axis length and the a-axis length after long-term cycling according to comparative example 1 and example 2.
[ Table 5]
Referring to fig. 10 and table 5, in the case of comparative example 1(a), after a long-term cycle of 2000 cycles, lithium ions in the crystal lattice disappeared, vacancies of lithium ions appeared in the lithium layer, and thus the c-axis length of the crystal structure was significantly increased, while the a-axis length was hardly changed. Therefore, in the crystal structure of the primary particles constituting comparative example 1, the arrangement of the primary particles constituting the secondary particles is broken due to the increase in the c-axis length, the passage of lithium ions is entirely blocked, and the electrochemical characteristics are deteriorated. On the other hand, in the case of example 2, the a-axis length and the c-axis length were hardly changed up to 2000 cycles and 5000 cycles because the shape and arrangement of the initial primary particles were maintained. In the case of example 2, during long-term cycling, lithium and nickel were exchanged with each other between a layer composed of only lithium and a layer composed of a transition metal containing nickel adjacent to each other, and thus lithium and the transition metal were located at sites corresponding to each other between the first mixed layer (a layer formed by site exchange between lithium and nickel in the lithium layer) and the second mixed layer (a layer formed by site exchange between nickel and lithium in the transition metal layer), thereby forming an ordered structure. The unit cell formed by the ordered structure includes a long-range ordered lattice having an increased a-axis lattice constant. The long-range ordered lattice is composed of a superlattice whose a-axis is twice as long as that of a lattice formed by the lithium layer and the transition metal layer, and thus the stability of the layered structure can be improved, thereby preventing structural collapse of the layered structure.
Fig. 11 depicts a graph (a) showing peaks on the (003) plane and peaks on the (104) plane in the positive electrode active material according to example 2, and a graph (b) showing the ratio between the peaks. Table 6 below shows the content of divalent nickel ions in the lithium layer, the peak values of the (104) plane, and the ratios between the peak values.
[ Table 6]
| Distinguishing | Comparative example 1 | Example 1 | Example 2 | Example 3 | Comparative example 2 |
| Content of F | 0mol％ | 0.5mol％ | 1mol％ | 2mol％ | 10mol％ |
| Ni of Li layer mountain 2+ (％) | 4.3 | 4.3％ | 4.7 | 4.8 | 6.8 |
| Ni in Li layer 2+ Rate of increase (%) | 1 | 1 | 1.1 | 1.13 | 1.63 |
| I (003) /I (104) | 1.74 | 1.74 | 1.72 | 1.71 | 1.59 |
| I (003) /I (104) Is reduced rate of | 1 | 1 | 0.99 | 0.98 | 0.92 |
Referring to fig. 11 and table 6, table 6 shows Ni in the first mixed layer (layer having nickel-substituted sites among layers composed of lithium only) 2+ Can be confirmed that as the F content increases, Ni in the first mixed layer increases 2+ Is increased and is taken as the peak value (I) on the (003) plane (003) ) Peak value (I) on the (104) plane (104) ) I of the ratio of (003) /I (104) The reduction rate of (a) is 1% or less. In particular, I (003) /I (104) Can be reduced by regular site exchange between nickel ions having an oxidation number of 2 and lithium ions. Further, the peak value (I) on the (003) plane (003) ) Peak value (I) on the (104) plane (104) ) I of the ratio of (003) /I (104) And may be 1.71 or less.
Fig. 12 shows XRD patterns according to example 2, example 3, comparative example 1 and comparative example 2.
Referring to fig. 12, it can be confirmed that the crystal structure of the positive electrode active material is not changed even when the formed ordered structure is increased due to the increase in the amount of fluorine added. This is because the ordered structure can form a certain regular lattice and thus maintain the layered structure of the positive electrode active material, and the lithium ions and nickel ions (oxidation number 2) formed in the first mixed layer or the second mixed layer have similar ionic radii.
Fig. 13 shows XPS charts according to example 2, example 3, comparative example 1, and comparative example 2. Fig. 14 shows Selected Area Electron Diffraction (SAED) of [010] band axes according to comparative examples 1 and 2. Fig. 15 shows High Angle Annular Dark Field (HAADF) images of [010] ribbon axes according to comparative example 1 and example 2. Table 7 below shows the nickel contents depending on the oxidation numbers of nickel oxidation for examples 1 to 3 and comparative examples 1 and 2.
[ Table 7]
Referring to fig. 13 and table 7, it can be determined that nickel includes Ni in terms of oxidation number 3+ And Ni 2+ And Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) Increases with increasing fluorine content. At this time, when Ni is present 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) Within a certain range, the ordered structure between the first mixed layer and the second mixed layer is formed in the layered structure of the positive electrode active material, and therefore, the ordered structure between the first mixed layer and the second mixed layer and thus the layered structure are more stably maintained without lithium ions being separated therefrom during long-term cycling. In XPS spectrum of Ni2P obtained by X-ray photoelectron spectroscopy (XPS), Ni is present at a binding energy of 850eV to 860eV 2+ Peak area of greater than Ni 3+ And Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3 + ) May be 49.7% to 130%. Furthermore, Ni is present at a binding energy of 850eV to 860eV 2+ Peak area and Ni 3+ The ratio of peak areas of (a) may be 0.497:1 to 1.3: 1.
Fig. 15 and 16 show electron diffraction patterns of [010] band axes according to example 2.
Referring to fig. 15, it can be determined that the positive electrode active material of example 2 has both a layered structure and an ordered structure. The layered structure is a structure in which lithium layers composed of only lithium and transition metal layers composed of only transition metals are alternately disposed. As shown in the left diagrams of fig. 15 and 16, only diffraction spots (peaks) in the (012), (014), and (003) space groups were found. In the ordered structure, since lithium and transition metal of the first mixed layer and the second mixed layer are site-exchanged, diffraction spots (peaks) that do not appear in the layered structure appear, and the intensity of these diffraction spots is lower than that of the diffraction spots that appear in the layered structure.
Specifically, in the ordered structure, the first mixed layer and the second mixed layer are stacked adjacent to each other and alternately and regularly repeated to form a layered structure. The electron diffraction pattern of the [010] ribbon axis (or [100] ribbon axis) of the ordered structure shows: a first diffraction spot group G1 in which one or more diffraction spots having a first intensity and corresponding to a crystal lattice formed by lithium layers and transition metal layers stacked adjacent to each other are aligned in one direction; and a second diffraction spot group G2 in which one or more diffraction spots having a second intensity relatively lower than the first intensity of the diffraction spots of the first diffraction group G1 and corresponding to a lattice formed by the first mixed layer and the second mixed layer stacked adjacent to each other are aligned in one direction.
The first and second diffraction spot groups G1 and G2 are alternately and regularly arranged with each other, and the first and second diffraction spot groups G1 and G2 may be spaced apart from each other at the same interval.
In the case of example 2, Ni 2+ And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) Increased by addition of fluorine and heat treatment, and thus Ni 2+ Site-exchanged with lithium of an adjacent layer composed of only lithium to form a first mixed layer and a second mixed layer. In the electron diffraction pattern, the layer composed of only lithium showed no diffraction spots, and only the transition metal layer showed diffraction spots in the form of white spots (upper right of fig. 12). On the other hand, when the first mixed layer and the second mixed layer are formed to form an ordered structure, diffraction spots are formed in a portion shown only in black to form a superlattice.
For example, in the first mixed layer, lithium and the transition metal may be alternately arranged, and in the second mixed layer, the transition metal and lithium may be alternately arranged. The lattice formed of the first mixed layer and the second mixed layer stacked adjacent to each other may include a superlattice, and the superlattice may have six lithium atoms and one transition metal atom.
Although the present invention has been described in detail with reference to the preferred embodiments, the scope of the present invention is not limited to the specific embodiments but should be defined by the appended claims. In addition, those skilled in the art will appreciate that many modifications and variations are possible without departing from the scope of the invention.
[ description of reference numerals ]
10: center part
20: surface portion
30: primary particles
100: secondary particles
Claims (17)
1. A positive electrode active material for a lithium secondary battery, the positive electrode active material having a layered structure and comprising lithium, a transition metal, fluorine (F), and oxygen,
wherein the layered structure includes a lithium layer composed of only lithium and a transition metal layer composed of only a transition metal including nickel,
wherein the nickel comprises Ni in terms of oxidation number 3+ And Ni 2+ And said Ni 2+ With said Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) Increases as the fluorine content increases.
2. The positive electrode active material according to claim 1, wherein the layered structure further comprises a first mixed layer and a second mixed layer each containing lithium and a transition metal, wherein the content of lithium in the first mixed layer is higher than the content of the transition metal, and the content of the transition metal in the second mixed layer is higher than the content of lithium.
3. The positive electrode active material according to claim 1, wherein the first mixed layer and the second mixed layer in the positive electrode active material are stacked adjacent to each other and are alternately and regularly repeated to form a layered structure, and the first mixed layer and the second mixed layer stacked adjacent to each other are configured such that the transition metal of the first mixed layer and the lithium of the second mixed layer correspond to each other.
4. The positive electrode active material according to claim 3, wherein the first mixed layer and the second mixed layer stacked adjacent to each other have an ordered structure, wherein the ordered structure is formed such that n1 lithium ions and n2 transition metal ions of the first mixed layer correspond to n1 transition metal ions and n2 transition metal ions, respectively, of the second mixed layer (wherein n1 and n2 are the same or different natural numbers), and unit cells formed by the ordered structure include a long-range ordered lattice having an increased a-axis lattice constant.
5. The positive electrode active material according to claim 3, wherein a crystal lattice formed of the first mixed layer and the second mixed layer stacked adjacent to each other includes a superlattice whose a-axis is twice as long as that of a crystal lattice formed of the lithium layer and the transition metal layer.
6. The positive electrode active material according to claim 1, wherein a peak (I) of a (003) plane in an X-ray diffraction spectrum of the positive electrode active material obtained using CuK α radiation is analyzed by XRD after an electrochemical reaction (003) ) Peak value (I) of and (104) plane (104) ) Ratio (I) of (003) /I (104) ) The reduction of (c) is less than 1%.
7. The positive electrode active material according to claim 6, wherein the peak value (I) of the (003) plane (003) ) The peak value (I) of the (104) plane (104) ) Said ratio (I) of (003) /I (104) ) Is 1.71 or less.
8. The positive electrode active material according to claim 1, wherein Ni is Ni at a binding energy of 850eV to 860eV in an X-ray photoelectron spectroscopy (XPS) spectrum of Ni2P obtained by XPS 2+ Peak area of greater than Ni 3+ And Ni 2 + And Ni 3+ Ratio of (Ni) 2+ /Ni 3+ ) Is 49 to 130 percent.
9. The positive electrode active material of claim 8 wherein Ni at a binding energy of 850eV to 860eV 2+ Peak area of (2) and Ni 3+ The ratio of the peak area of (a) is from 0.49:1 to 1.3: 1.
10. The positive electrode active material according to claim 1, wherein the positive electrode active material comprises a secondary particle composed of a group of a plurality of primary particles, and at least one of the primary particles includes a grain coating comprising fluorine at a grain boundary between the primary particles.
11. The positive electrode active material according to claim 1, wherein the positive electrode active material is represented by the following formula 1:
[ formula 1]
Li 1-x M 1-y [Li x M y ]O 2-z F z
Wherein x + y is 1; z is more than or equal to 0.005 and less than or equal to 0.02; and M is any one of: ni; ni and Co; ni and Mn; ni, Co and Mn; ni and Al; ni, Co and Al; ni, Mn and Al; and Ni, Co, Mn and Al.
12. The positive electrode active material according to claim 1, wherein
The layered structure further includes a first mixed layer and a second mixed layer each containing lithium and a transition metal;
the content of lithium in the first mixed layer is higher than that of the transition metal, and the content of the transition metal in the second mixed layer is higher than that of lithium;
the first mixed layer and the second mixed layer in the positive electrode active material are stacked adjacent to each other and alternately and regularly repeated to form the layered structure; and
the electron diffraction pattern of the [010] or [100] crystallographic band axis of the layered structure shows:
a first diffraction spot group in which one or more diffraction spots having a first intensity and corresponding to a crystal lattice formed by the lithium layer and the transition metal layer stacked adjacent to each other are aligned in one direction; and
a second diffraction spot group in which one or more diffraction spots having a second intensity lower than the first intensity of diffraction spots of the first diffraction group and corresponding to a lattice formed by the first mixed layer and the second mixed layer stacked adjacent to each other are aligned in one direction.
13. The positive electrode active material according to claim 12, wherein the first diffraction spot group and the second diffraction spot group are alternately and regularly arranged with each other, and the first diffraction spot group and the second diffraction spot group are spaced apart from each other.
14. The positive electrode active material according to claim 1, wherein
The layered structure further includes a first mixed layer and a second mixed layer each containing lithium and a transition metal;
the content of lithium in the first mixed layer is higher than that of the transition metal, and the content of the transition metal in the second mixed layer is higher than that of lithium;
the lithium and the transition metal in the first mixed layer are alternately arranged;
the transition metal and lithium in the second mixed layer are alternately arranged; and
the crystal lattice formed by the first mixed layer and the second mixed layer stacked adjacent to each other includes a superlattice.
15. The positive electrode active material according to claim 1, wherein the primary particles include rod-like particles formed into a plate-like shape having a long axis and a short axis in cross section, and the rod-like particles are oriented such that the long axis of the rod-like particles faces a central portion of the secondary particles.
16. The positive electrode active material of claim 17 wherein the transition metal comprises any one or more of nickel (Ni), manganese (Mn), and cobalt (Co), wherein at least one of the transition metals has a concentration gradient in at least a portion of the secondary particles from a center of the secondary particles toward a surface of the secondary particles.
17. A lithium secondary battery comprising:
a positive electrode comprising the positive electrode active material for a lithium secondary battery according to any one of claims 1 to 16;
a negative electrode facing the positive electrode and composed of graphite or lithium metal;
a separator between the positive electrode and the negative electrode; and
an electrolyte solution or a solid electrolyte containing a lithium salt.
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| Publication number | Publication date |
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| KR102545888B1 (en) | 2023-06-22 |
| EP4084137A1 (en) | 2022-11-02 |
| EP4084137A4 (en) | 2024-01-24 |
| KR20210083202A (en) | 2021-07-06 |
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